Within the IP (Internet Protocol) Multimedia Subsystem (IMS) as defined by 3rd Generation Partnership Project (3GPP) Session Initiation Protocol (SIP) defined by Internet Engineering Task Force (IETF) is used for controlling communication. SIP is an application-layer control protocol for creating, modifying, and terminating sessions with one or more participants. These sessions may include Internet multimedia conferences, Internet telephone calls, and multimedia distribution. Members in a session can communicate via multicast or via a mesh of unicast relations, or a combination of these. Session Description Protocol (SDP) is a protocol which conveys information about media streams in multimedia sessions to allow the recipients of a session description to participate in the session. The SDP offers and answers can be carried in SIP messages. Diameter protocol has been defined by IETF and is intended to provide an Authentication, Authorization and Accounting (AAA) framework for applications such as network access or IP mobility.
New services are regularly developed and implemented in communication networks for improving quality or user experience. One example for such a new service is the so-called customized alerting tones (CAT) which will be supported, for example, by IP Multimedia Subsystem (IMS). The CAT service is an operator specific service by which an operator enables a subscriber to customize, for example, the media which is played to the calling party during alerting of the called party.
For the implementation of such new services like CAT, there are several mechanisms available. One of these mechanisms is the “early session model”. In the early session (or early media) model, mechanisms for exchanging media (e.g., audio and video) before a particular (final) session is accepted by the called user are discussed. Within a dialog, early media occurs from the moment the initial INVITE is sent until a final response is generated. Examples of early media generated by the callee are ringing tone and announcements (e.g., queuing status). An offer/answer exchange is used where a so-called disposition type of the session descriptions is indicated as “session” (for final session setting) or “early session” (for early session setting), depending on which session type the respective offer/answer is related to.
Conventionally, early media are established in the same way as regular media. That is, an offer/answer exchange is used where a so-called disposition type of the session descriptions is indicated as “session” (for final session setting). This way of establishing early media sessions is also known as gateway model.
Towards the access network, Policy and Charging control (PCC) is used to authorize or reserve transmission resources, e.g. in general packet radio service (GPRS) or long term evolution (LTE) packet networks and the attached radio networks, and configure service specific charging for the services negotiated via the IMS. The IMS Proxy Call Session Control Function (P-CSCF) conveys service information derived from the session description information (SDP) within the IMS signalling via so-called Rx interface towards the Policy and Charging Rules Function (PCRF), that in turn can derive PCC rules and possibly QoS Rules from this information and policy configured by the operator, and can convey these PCC rules towards the Policy and Charging Enforcement Function (PCEF), such as gateway GPRS support node (GGSN), over so-called Gx interface and the QoS rules towards the Bearer Binding and Event Reporting Function (BBERF). The PCEF and/or BBERF can interact with other packet network nodes to authorize resource requests from the User Equipment (UE) or to set up or configure transmission resources towards the LIE. The PCEF can also control the IP flows of user plane data passed through it, only passing flows that are authorized by PCC rules and collecting charging related data related to those PCC rules.
However, reactions of a P-CSCF not supporting the early-session disposition type and receiving such SDP are undefined in standards, but behaviour leading to blocking of media or call failure may happen, such as:                The P-CSCF could reject such call setups.        The P-CSCF could also intermingle the SDP for early session and final session, possible resulting in wrong filters to be installed and wrong QoS to be provisioned in the PCEF for the early or final session, and thus in the blocking of early and/or final media flow.        The P-CSCF could also ignore the early-session disposition SDP, thus preventing a configuration of the PCEF to support them, resulting in a blocking of the early media streams.        